This invention relates to a process for the production of moldings or films based on polyisocyanate polyadducts containing urea groups and, optionally, urethane groups by thermoplastic molding. The polyisocyanate polyadducts are based on compounds of relatively high functionality containing isocyanate-reactive atoms and having a high urea content.
Thermoplastic polyurethane elastomers are know. See, for example, Becker/Braun, Kunststoff-Handbuch, Vol. 7, "Polyurethane", Carl Hanser Verlag, Munich/Vienna (1983), pages 428 et seq. The previously known thermoplastically processible polyurethane elastomers are based on diisocyanates, relatively high molecular weight dihydroxy compounds (more especially polyester diols of relatively high molecular weight), and low molecular weight diols as chain-extending agents. Such polyurethanes reportedly have a linear structure; i.e. they do not contain crosslinking or branching sites in the molecule. The use of diamines as chain-extending agents in the production of thermoplastically processible polyurethane elastomers has also been disfavored in the past (see the above-cited handbook, page 428, section 8.2.1), because urea groups which are regarded as hard segments that create an obstacle to thermoplastic processing are incorporated into the molecule when diamine chain-extending agents are used.
Until now it has been assumed that moldings produced by reaction injection molding would not lend themselves to thermoplastic processing because of the use of compounds having relatively high molecular weight and relatively high functionality and containing isocyanate-reactive hydrogen atoms, particularly when used in the presence of high concentrations of incorporated urea groups. For this reason, closed molds corresponding in shape to the shape of the desired end product (for example, automobile fenders) have been used for the production of elastomers.
However, it has now surprisingly been found that even polyisocyanate polyadducts containing urea groups in greater numbers than the urethane groups, and which in addition have been prepared using branched, relatively high molecular weight synthesis components containing isocyanate-reactive groups, lend themselves to thermoplastic processing. It has also been found that the favorable mechanical properties of these polyisocyanate polyadducts are not adversely affected by thermoplastic molding, irrespective of whether the polyisocyanate polyadducts contain fillers and/or reinforcing materials.
The process according to this invention makes it possible to mold elastomeric polyisocyanate polyadducts, which correspond chemically to the above-mentioned products of the prior art, by thermoplastic molding. Thus, there is no longer any need for molding during production using molds corresponding to the desired end product.